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Magnetite (Fe3O4) is attracting much interest in the last years due to its robust ferrimagnetism down to nanometer thickness, good electrical conductivity and presumed half-metal character. In particular, Fe3O4 films are studied as ideal cases for the design of improved bulk magnets [1] and have been tentatively used in spin-valves and spin-LEDs. Fe3O4 presents a low-temperature metal-insulator transition, the Verwey transition (TV) which has also been proposed for spintronic applications. An open question is to what extent the preparation of Fe3O4 films can affect their detailed magnetic properties, such as the magnetic anisotropy axis. This information is required to efficiently apply Fe3O4 in technological multiphase magnets and spintronic applications [1]. Most of studies dealing with bulk and Fe3O4 thin film systems show room temperature (RT) in-plane <110> magnetic easy axis. By contrast, we show in this work the preparation of pure stoichiometric Fe3O4 thin films with RT easy axes along the in-plane <100> directions [2], i.e. rotated by 45º respect to previous studies. Fe3O4 films have been grown by ablation from a sintered hematite target using a nanosecond infrared (IR) laser at 1064 nm and a substrate temperature of 750 K [3]. Single crystal substrates of SrTiO3, MgAl2O4 and MgO have been used. The films were characterized using XRD, AFM, Raman and Mössbauer spectroscopies, vectorial magneto-optical Kerr effect microscopy (v-MOKE) and SQUID magnetometry. All films consisted of stoichiometric Fe3O4 and presented a Verwey transition at TV=115-118 K. RT in-plane hysteresis loops were measured by vectorial-MOKE as a function of the direction of the applied magnetic field in the 0º-360º range with an angular step of 5º. For all epitaxial films under study, the highest coercivity and remanence are found at 0º, 90º, 180º and 270º (i.e. <100> directions), thus orthogonal to each other, while the lowest coercivity values are found between them [Figures 1(a) and 1(b

The interface between two different oxides has properties different from the ones corresponding to the constituent layers in bulk. Different orders can arise due to the complexity of these materials in which the orbital degree of freedom, magnetism and lattice are strongly interdependent. Here we present a joint theoretical-experimental effort to understand the properties of a multilayer formed by a metallic ferromagnetic manganite oxide (La0.7Sr0.3MnO3) and the insulating SrTiO3. Magnetoresistance measurements as a function of the relative angle between the magnetic field and the interface plane have shown an unexpected in-plane peak. Calculations of resistivity in a model system including spin-orbit coupling reveal that the unexpected in-plane maximum is due to transport through a two-dimensional ferromagnetic electron gas formed by orbital reconstruction at the manganite interface. These orbital and magnetic reconstructions are supported by X-ray linear dichroism and ab-initio calculations.

The layered rare-earth diantimonides RSb2 are anisotropic metals with generally low electronic densities whose properties can be modified by substituting the rare earth. LaSb2 is a nonmagnetic metal with a low residual resistivity presenting a low-temperature magnetoresistance that does not saturate with the magnetic field. It has been proposed that the latter can be associated to a charge density wave (CDW), but no CDW has yet been found. Here we find a kink in the resistivity above room temperature in LaSb2 (at 355 K) and show that the kink becomes much more pronounced with substitution of La by Ce along the La1-xCexSb2 series. We find signatures of a CDW in x-ray scattering, specific heat, and scanning tunneling microscopy (STM) experiments in particular for x≈0.5. We observe a distortion of rare-earth-Sb bonds lying in-plane of the tetragonal crystal using x-ray scattering, an anomaly in the specific heat at the same temperature as the kink in resistivity and charge modulations in STM. We conclude that LaSb2 has a CDW which is stabilized in the La1-xCexSb2 series due to substitutional disorder.

The design of artificial vortex pinning landscapes is a major goal toward large scale applications of cuprate superconductors. Although disordered nanometric inclusions have shown to modify their vortex phase diagram and to produce enhancements of the critical current (MacManus-Driscoll, J. L.; Foltyn, S. R.; Jia, Q. X.; Wang, H.; Serquis, A.; Civale, L.; Maiorov, B.; Hawley, M. E.; Maley, M. P.; Peterson, D. E. Nat. Mater. 2004, 3, 439?443 and Yamada, Y.; Takahashi, K.; Kobayashi, H.; Konishi, M.; Watanabe, T.; Ibi, A.; Muroga, T.; Miyata, S.; Kato, T.; Hirayama, T.; Shiohara, Y. Appl. Phys. Lett. 2005, 87, 1-3), the effect of ordered oxide nanostructures remains essentially unexplored. This is due to the very small nanostructure size imposed by the short coherence length, and to the technological difficulties in the nanofabrication process. Yet, the novel phenomena occurring at oxide interfaces open a wide spectrum of technological opportunities to interplay with the superconductivity in cuprates. Here, we show that the unusual long-range suppression of the superconductivity occurring at the interface between manganites and cuprates affects vortex nucleation and provides a novel vortex pinning mechanism. In particular, we show evidence of commensurate pinning in YBCO films with ordered arrays of LCMO ferromagnetic nanodots. Vortex pinning results from the proximity induced reduction of the condensation energy at the vicinity of the magnetic nanodots, and yields an enhanced friction between the nanodot array and the moving vortex lattice in the liquid phase. This result shows that all-oxide ordered nanostructures constitute a powerful, new route for the artificial manipulation of vortex matter in cuprates.

Magnetic nanobeads are synthesized by coprecipitation of hollow iron oxide nanoparticles and an amphiphilic polymer. The resulting nanobeads can be tuned in diameter and nanoparticle content. X-ray absorption near-edge structure (XANES) spectroscopy and superconducting quantum interferometer device (SQUID) characterization of the nanobeads reveal that they exhibit an increased effective magnetic anisotropy as compared to the individual nanoparticles, despite that no structural changes of the particles had occurred during the embedding process into the polymer. The spin-spin relaxation times of the pristine hollow nanoparticles and of the final magnetic nanobeads reveal a high R2 relaxivity of 206 s-1 mM-1 for the magnetic nanobeads. This result should enable their application as negative contrast enhancing agents in magnetic resonance imaging.

We have determined why hydrothermal synthesis is not an adequate method to obtain nanoparticles in the Sr-Mn-O system. At high KOH concentration, which is more appropriate for obtaining nanoparticles, Sr is partially incorporated inside an intermediate (K,Sr)xMnO2·nH2O layered phase that prevents the formation of 4H-nanoparticles. Numerous experiments have been performed, screening the parameters involved in such processes (precursors, pH, temperature, etc.) in order to understand their influence on the shape, size, and purity of the 4H-final product. On analyzing the experimental results obtained by XRD, TEM, TGA, and EDS, among others, some peculiarities of the synthetic procedure were detected, including the appearance of side phases, SrMn3O6–x nanoribbons or the above-mentioned birnessite-type (K,Sr)xMnO2·nH2O phase, and the insertion of K+ within the 4H-phase prepared at high KOH concentration. Magnetic features are also discussed.

A truly developed carbon-based nanoelectronics requires, among other features, low price, large production scale, large domain area and high crystalline quality. Epitaxially grown graphene on silicon carbide (SiC) can meet all of these requirements[1]. Moreover, functionalization of graphene is expected to be an important step for development of graphene-based materials with tailored features, due to the possible control of optical and electronic properties[2,3]. However, the high chemical inertness of graphene makes it difficult a covalent and controlled functionalization with organics. Most of the works performed until now involve either controlled adsorption of low-interacting molecular structures or unruly functionalization. In this work, we target to develop new strategies for controlled covalent anchoring of the organic molecules to the graphene lattice that can be used either to modify their properties or as a link for anchoring larger nanostructures. In our experiment, high quality graphene was epitaxially grown on a 4H-SiC(0001) substrate with n-type doping character by chemical vapor deposition (CVD) at 1600oC under an argon (Ar) laminar flow in a hot-wall Aixtron VP508 reactor. The graphene produced by this method is much less sensitive to SiC surface defects, resulting in higher electron mobility than those grown by Si sublimation process. For epitaxial CVD growth, we formed an Ar boundary layer thick enough to prevent Si sublimation, but allowing for the diffusion of propane gas that was led into the reactor as the precursor which enables the controlled synthesis of a precisely determined number of graphene layers. The functionalization protocols have been performed in ultra high vacuum, and we have chosen p-aminophenol organic molecules because they include two functional groups: amine and hydroxyl, allowing us to determine which one is more prone to make a bond with graphene. Prior and upon adsorption, the surfaces have been studied in-situ using seve

The production of high-quality graphene inexpensively and in bulk is an absolutely necessary first step for the material to ever live up to its promise in commercial applications. Among the different reported growth methods, chemical vapor deposition (CVD) and its variants with transition metal substrates has proven its ability to produce large-scale, one-atom thick graphene sheets [1-2]. Particularly attractive is the use of low carbon solubility Cu substrates for CVD graphene growth, owing to its inexpensiveness and the possibility of post-growth graphene transfer on arbitrary substrates [3]. In this work we present the growth of graphene layers by chemical vapor deposition under ultra-high vacuum conditions on polycrystalline oxygen-free Cu foils. As a carbon source, a C60 evaporator maintained at 500 C has been used. Prior to carbon evaporation the Cu foils have been treated by Ar-sputtering and thermal annealing cycles in order to clean them and promote the growth of well oriented large Cu terraces, especially suitable for LEED analysis (figure 1). C60 deposition has taken place while controlling the Cu foil temperature with an optical pyrometer. After growth is complete, sample analysis is performed with different techniques to characterize the graphene layer. In-situ LEED images show well defined Cu 111 and 100 reflections and rings corresponding to graphene in various orientations with respect to the Cu grains (figure 1). Ex- situ Atomic Force Microscopy (AFM) and Raman spectroscopy are employed to gather information on sample morphology and quality (figure 1). In order to determine the electronic band structure, angle resolved photoelectron emission measurements have been done in synchrotron facility where linear behavior of electrons near Dirac point has been observed (figure 1). We are currently optimizing graphene transfer from our samples to insulating oxide substrates with aim to determining its bandgap and macroscopic and local magnetotransport proper

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

© 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim. A route for fabricating structurally perfect cobalt ferrite magnetic nanostructures is demonstrated. Ultrathin islands of up to 100 ¿m2 with atomically flat surfaces and free from antiphase boundaries are developed. The extremely low defect concentration leads to a robust magnetic order, even for thicknesses below 1 nm, and exceptionally large magnetic domains. This approach allows the evaluation of the influence of specific extrinsic effects on domain wall pinning.

New magnetic bio-hybrid matrices for potential application in drug delivery are developed from the assembly of the biopolymer alginate and magnetic graphite nanoparticles. Ibuprofen (IBU) intercalated in a Mg-Al layered double hydroxide (LDH) was chosen as a model drug delivery system (DDS) to be incorporated as third component of the magnetic bionanocomposite DDS. For comparative purposes DDS based on the incorporation of pure IBU in the magnetic bio-hybrid matrices were also studied. All the resulting magnetic bionanocomposites were processed as beads and films and characterized by different techniques with the aim to elucidate the role of the magnetic graphite on the systems, as well as that of the inorganic brucite-like layers in the drug-loaded LDH. In this way, the influence of both inorganic components on the mechanical properties, the water uptake ability, and the kinetics of the drug release from these magnetic systems were determined. In addition, the possibility of modulating the levels of IBU release by stimulating the bionanocomposites with an external magnetic field was also evaluated in in vitro assays.

There is an increasing interest in the generation of well-defined nanoparticles (NPs) not only because of their particular properties resulting from their reduced dimensions, but also because they are promising building blocks for more complex materials in the fast growing nanotechnology [1]. As a consequence, the development of fabrication methods of high quality NPs is a key issue to follow the increasing demand of complex multifunctional nanoparticles for advanced applications [2]. We will present a bottom-up fabrication route based on the sputtering gas aggregation source that allows the generation of nanoparticles with controllable and tunable chemical composition and structure while keeping the control of their size. This technique, called Multiple Ion Cluster Source (MICS) [3], is an evolution of standard Ion Cluster Sources (ICS) [4]. Through examples, we will show that, apart from the generation of alloyed nanoparticles [5], the technique allows the generation of core-shell and core@shell@shell nanoparticles. All these possible combinations are generated in one single step process and under ultra-high vacuum (UHV) conditions, which leads to the formation of NPs with high purity.

The integration of ferroelectric oxides in microelectronic devices is usually hampered by the degradation of their properties in thin film form. For example, films based on solid solutions with excellent piezoelectric properties around their Morphotropic Phase Boundaries, such as Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMNT) or the lead free (Bi1/2Na1/2)TiO3-BaTiO3 (BNBT), show a drastic decrease of the remnant polarization respect to their bulk counterparts. The decrease of the grain size associated to the reduced thickness has been reported as the main cause of this effect in polycrystalline films. The stabilization of relaxor-like domains in submicrometric grains reported in ceramics makes that, as it happens in relaxor ferroelectrics, the removal of the electric field results in the disappearance of the polarization, which reduces drastically all remnant properties. A solution is the combination of the low remnant film with hard ferroelectric layers in multilayer composite films. The hard ferroelectric layers, when poled, induce an internal bias in the low remnant film, which therefore maintains the good properties of this film when the electric field is removed. In this work a study of the properties of BNBT and BiFeO3 (BFO) multilayer composite films is shown. Not only it allows taking advantage of the excellent properties of BNBT, but also solves part of the problem of the large leakage currents of multiferroic BFO, which supports a possible magneto-electric functionality.

Complex core@shell and core@shell@shell nanoparticles are systems that combine the functionalities of the inner core and outer shell materials together with new physico-chemical properties originated by their low (nano) dimensionality. Such nanoparticles are of prime importance in the fast growing field of nanotechnology as building blocks for more sophisticated systems and a plethora of applications. Here, it is shown that although conceptually simple a modified gas aggregation approach allows the one-step generation of well-controlled complex nanoparticles. In particular, it is demonstrated that the atoms of the core and the shell of the nanoparticles can be easily inverted, avoiding intrinsic constraints of chemical methods.

Herein a facile and versatile one step synthesis of magnetite nanoparticles coated with polydopamine is described. Magnetite nanoparticles are synthesized electrochemically by electrooxidation of iron in an aqueous medium in the presence of dopamine. The oxidative conditions and alkaline pH involved in the synthesis favor the self-polymerization of dopamine that adheres at the surface of the magnetic nanoparticles in a simultaneous process. It is shown that the size of the magnetite nanoparticles as well as the polydopamine coating can be controlled by varying the synthetic approach that is, adding dopamine at the beginning of the electrosynthesis, in the middle or at the end of the process. The particle size of the core varies between a few nanometers and 25 nm while the shell can reach thicknesses of up to ∼5 nm. The obtained hybrid nanoparticles were characterized by thermogravimetric analysis (TGA), infrared spectroscopy (FTIR), X-ray diffraction (XRD) and transmission electron microscopy (TEM). In addition, the magnetic measurements of the different obtained materials were carried out showing a variety of magnetic behaviors depending on the synthetic procedure.

In this paper, we explore the interfacial effects appearing in highly strained La0.7Ca0.3MnO3 (LCMO) ultra-thin films (10-12nm) grown on BaTiO3 (BTO) ferroelectric substrates. The strong tendency to phase separation of this optimally doped manganite contributes to the exotic phenomena observed in magnetism and transport experiments: the so-called Matteucci magnetic loops, magnetic granularity and a second metal insulator transition are observed between 50K and the LCMO Curie temperature, 180K. All these properties define the multiferroic character of these heterostructures, which in LCMO//BTO system is strongly linked to magnetoelastic coupling.

Electric-field control of magnetism has remained a major challenge which would greatly impact data storage technology. Although progress in this direction has been recently achieved, reversible magnetization switching by an electric field requires the assistance of a bias magnetic field. Here we take advantage of the novel electronic phenomena emerging at interfaces between correlated oxides and demonstrate reversible, voltage' driven magnetization switching without magnetic field. Sandwiching a non-superconducting cuprate between two manganese oxide layers, we find a novel form of magnetoelectric coupling arising from the orbital reconstruction at the interface between interfacial Mn spins and localized states in the CuO 2 planes. This results in a ferromagnetic coupling between the manganite layers that can be controlled by a voltage. Consequently, magnetic tunnel junctions can be electrically toggled between two magnetization states, and the corresponding spin' dependent resistance states, in the absence of a magnetic field.

We report a study of the morphological, chemical, structural and magnetic properties of FeB nanoparticles (NPs) grown using an ion cluster source. A large size distribution of NPs with the mean size of dominant population around 10-14 nm is obtained. The structural characterization confirms the spherical shape and the amorphous nature of the FeB NPs. The evolution of the chemical composition of the NPs in depth is analyzed by means of x-ray photoelectron spectroscopy. A mixture of Fe and B oxides and oxynitrides surrounds a core where metallic Fe is present. According to the magnetic measurements, this core size is close to 3 nm. The majority of NPs are superparamagnetic at room temperature and the blocking temperature was found to be 17.5 K. The outer shell thickness of more than 7 nm formed by oxides and oxynitrides inhibits the magnetic interaction between NPs resulting in an almost negligible coalescence, making these NPs suitable to be used in biomedical applications.

Fe65Co35 alloys are technologically relevant, especially in magnetic storage and composite permanent magnets, due to the fact that they present higher saturation magnetization per volume than any other material. Out of the various approaches undertaken for its production, mechanical ball milling remains the most common and efficient method, especially considering the large industrial scale of the applications. With the development of cost-efficient processing in mind, the influence of performing the synthesis of the FeCo alloys in air instead of the standard argon atmosphere is studied. The structural and magnetic characterization, along with the study of the oxygen content of the samples, proves that synthesizing FeCo alloys in air produce materials with nearly identical magnetic performance as their argon-milled counterpart, with the oxidation extent of the materials consisting almost exclusively of the oxide passivating layer located at the surface. In addition, no aging effect was observed in the saturation magnetization up to 6 months. It is concluded that the use of argon atmospheres, desiccators and/or glove boxes may be entirely removed from the process without affecting the magnetic properties.

The magnetoresistance of La0.7Sr0.3MnO3/SrTiO3 superlattices with magnetic field rotating out‐of‐plane shows unexpected peaks for in‐plane fields. Resistivity calculations with spin–orbit coupling reveal that orbital reconstruction at the manganite interface leads to a 2D ferromagnetic electron gas coupled antiparallel to the manganite “bulk”. These orbital and magnetic reconstructions are supported by X‐ray linear dichroism and ab initio calculations.

© 2014 AIP Publishing LLC. Abundant efforts are being devoted to the production of nanostructured composites based on ferrites where the exchange-spring mechanism yields larger energy products. However, experimental results occasionally contradict the predictions, and the origin of the phenomenology is not always thoroughly understood. Magnetic properties may eventually evolve in trends similar to that of exchange-spring magnets due to structural changes unrelated to exchange-coupling mechanisms at hard/soft interfaces. Thus, the evaluation of the possible sources of magnetic improvement is pivotal in determining and optimizing the extent of exchange-coupling in spring magnets. Here, we report the observation of remanence improvement and single-phase reversal behavior at room-temperature in CoFe2O4/FeCo composites, where the hard and soft phases are predominantly uncoupled. The results are explained in terms of the temperature evolution of the individual properties of CoFe2O4. Causes for the observation of single-phase magnetization reversal in exchange-uncoupled systems are discussed. Our results indicate that these type of phenomena must be taken into account in order to thoroughly understand the optimization required to improve the magnetic performance of CoFe2O4-based composites.

Stoichiometric 4H-SrMnO3.0 nanoparticles have been synthesized from thermal decomposition of a new molecular heterometallic precursor [SrMn(edta)(H2O)5]·3/2H 2O whose crystal structure has been solved by single crystal X-ray diffraction. From this precursor, highly homogeneous 4H-SrMnO3.0 nanoparticles, with average particle size of 70 nm, are obtained. The agglomeration of these nanoparticles maintains the sheet-assembling morphology of the metal-organic compound. Local structural information, provided by atomically resolved microscopy techniques, shows that 4H-SrMnO3.0 nanoparticles exhibit the same general structural features as the bulk material, although structural disorder, due to edge dislocations, is observed. The nanometric particle size enables a topotactic reduction process at low temperature stabilizing a metastable 4H-SrMnO2.82 phase. The oxygen deficiency is accommodated through extra cubic layers breaking the...chch... 4H-sequence. These defect areas are Mn3+ rich, as evidenced by high energy resolution EELS data. Magnetic characterization of nano-SrMnO 3.0 shows significant variations with respect to the bulk material. Besides the dominant antiferromagnetic interactions, a weak ferromagnetic contribution as well as exchange bias and a glassy-like component are present. After the reduction process, the stabilization of Mn3+ in the 4H-structure gives rise to magnetic anomalies in the 40-60 K temperature range. The origin of such magnetic features is discussed. © 2014 American Chemical Society.

The effect of composition and particle size on the electrical and magnetic behavior of ZnxCo1-xFe2O4 spinels (x = 0.2 and 0.3) has been studied. Powdered samples of these ferrites have been synthetized by the liquid mix technique and hydrothermal method (in sub and supercritical conditions), leading to average particle sizes of ca. 50 and 10 nm, respectively. They have been characterized by means of X-ray diffraction, Thermogravimetric analysis, Energy-Dispersive X-ray Spectroscopy and impedance and magnetic measurements. Permittivity values up to ca. 500 were registered at 375 K, which remained almost constant at moderate frequencies, between 10 3 and 106 Hz. Stabilization of polarization phenomena is very sensitive to grain size and composition. Dielectric behavior evolves to a relaxor ferroelectric response when grain size becomes nanometric and, particularly, when the sample shows high monodispersion. The conduction mechanism and type of majority charge carriers have been established from Seebeck measurements. The x = 0.3 sample, prepared in supercritical water for the first time, exhibits homogeneous particle size distribution, superparamagnetic behavior and Curie temperature lower than those corresponding to similar microsized samples. The electronic response of the ferrites obtained in supercritical conditions is interpreted considering the possible short scale polarization of nanodomains. © 2014 Elsevier B.V. All rights reserved.

We report the experimental observation of a submicron cellular structure on the surface of silicon targets eroded by an ion plasma. Analysis by atomic force microscopy allows us to assess the time evolution and show that the system can be described quantitatively by the convective Cahn-Hilliard equation, found in the study of domain coarsening for a large class of driven systems. The space-filling trait of the ensuing pattern relates it to evolving foams. Through this connection, we are actually able to derive the coarsening law for the pattern wavelength from the nontrivial topological dynamics of the cellular structure. Thus, the study of the topological properties of patterns in nonvariational spatially extended systems emerges as complementary to morphological approaches to their challenging coarsening properties. © 2014 American Physical Society.

Discoveries of room-temperature ferromagnetism (RTFM) in semiconductors hold great promise in future spintronics technologies. Unfortunately, this ferromagnetism remains poorly understood and the debate concerning the nature, carrier-mediated versus defect-mediated, of this ferromagnetism in semiconducting oxides is still open. Here, by using X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD), it is demonstrated that the oxygen ions have a ferromagnetic response in different ZnO-based compounds showing RTFM behavior: ZnO nanoparticles capped with organic molecules and ZnO/ZnS heterostructures. These results demonstrate the intrinsic occurrence of RTFM in these systems, and point out that it is not related to the metallic cation but it relays on the conduction band of the semiconductor.

Thin film multiferroic nanocomposites might enable a range of potentially disruptive integrated magnetoelectric devices for information storage, spintronics, microwave telecommunications, and magnetic sensing. With this aim, we have investigated ion implantation of magnetic species into ferroelectric single crystal targets as a radically novel approach to prepare film nanoparticulate magnetic-metal ferroelectric-oxide composites. These materials are an alternative to multiferroic oxide epitaxial columnar nanostructures that are under intensive research, but whose magnetoelectric response is far from expectations. Here, we unambiguously demonstrate the preparation of such a thin film multiferroic nanocomposite of Co and BaTiO3 by ion implantation of a high dose of the magnetic species, followed by rapid thermal processing under tailored conditions. Results thus constitute a proof of concept for the feasibility of obtaining the materials by this alternative approach. Ion implantation is a standard technique for the microelectronic industry in combination with well-established patterning procedures. © 2014 American Chemical Society.

The production of high-quality inexpensive graphene is an absolutely necessary first step for the material to ever live up to its promise in commercial applications. Among the different growth methods reported to date, physical vapor deposition (PVD) from a suitable organic precursor emerges as an advantageous procedure since lower substrate temperatures are required to produce graphene. On the other hand, particularly attractive is the use of low carbon solubility Cu substrates for graphene growth, owing to its inexpensiveness and the possibility of post-growth graphene transfer on arbitrary substrates.

Magnetite (Fe3O4) films grown by pulsed laser deposition (PLD) are usually prepared with ultraviolet lasers. This is presumably done to minimize the presence of particulates on the films which might result in detrimental magnetic properties. However, inertia seems to be the rule followed when choosing the preparation parameters in a vast number of published studies. A systematic and comprehensive study on the influence of the laser wavelength on the microstructure, morphology and magnetic properties of Fe3O4 films is missed. Furthermore, and despite of magnetite being a material extensively studied in the shape of bulk and films, a rigorous study of the angular evolution of the magnetic properties in dependence of the measuring temperature for magnetite films grown on different substrates is still due. In this study high quality stoichiometric magnetite thin films of about 50 nm have been grown by ablation from a self-prepared sintered hematite target using a nanosecond infrared (IR) laser at 1064 nm and a substrate temperature of 475ºC. Single crystal substrates of SrTiO3:Nb, MgAl2O4 and MgO have been used in the preparation of Fe3O4 epitaxial films. By comparison, polycrystalline Fe3O4 films were grown on Al2O3 and Si substrates. The films were characterized using XRD, AFM, Raman and Mössbauer spectroscopy, vectorial magneto-optical Kerr effect microscopy (v-MOKE) and SQUID magnetometry. All films consisted of stochiometric magnetite and presented a Verwey transition at TV=115-118 K. Room temperature (RT) coercivity values (35 – 45 mT) agree well with values reported in the literature for magnetite films in this thickness range. In-plane hysteresis loops were measured in the temperature range 90-300 K by v-MOKE in dependence of the orientation of the applied magnetic field in the 0-360º range (angular step of 4.5º). For all epitaxial films under study, the highest RT coercivity is found at 0, 90, 180 and 270º, thus orthogonal to each other, while the lowest coerci

Los materiales multiferroicos han cobrado un interés creciente en los últimos años dada la potencialidad de sus aplicaciones prácticas. Los materiales basados en BiFeO3 pueden considerarse los más estudiados hasta la fecha dentro de los pocos multiferroicos conocidos. No obstante, sus propiedades aún no son adecuadas para su empleo en dispositivos pues suelen presentar una elevada conductividad eléctrica y/o una respuesta antiferromagnética. La modificación de estos materiales mediante dopado con Ti4+ puede disminuir las pérdidas dieléctricas y proporcionar un comportamiento ferromagnético. Sin embargo, los cambios producidos en la estructura, microestructura y propiedades de los materiales de BiFeO3 como consecuencia del dopado con Titanio no están claros. En este trabajo se presenta una caracterización detallada de cerámicas de composición nominal BiFe0.95Ti0.05O3 con una elevada resistencia y una respuesta magnética extraordinaria. Los resultados evidencian la formación de una nanoestructura relacionada con la segregación del dopante en borde de grano que es la responsable de los cambios producidos en las propiedades magnetoeléctricas de estos materiales.

We present measurements of the superconducting critical temperature T c and upper critical field Hc2 as a function of pressure in the transition metal dichalcogenide 2H-NbS2 up to 20 GPa. We observe that Tc increases smoothly from 6 K at ambient pressure to about 8.9 K at 20 GPa. This range of increase is comparable to the one found previously in 2H-NbSe2. The temperature dependence of the upper critical field Hc2(T) of 2H-NbS2 varies considerably when increasing the pressure. At low pressures, Hc2(0) decreases, and at higher pressures both Tc and Hc2(0) increase simultaneously. This points out that there are pressure induced changes of the Fermi surface, which we analyze in terms of a simplified two-band approach.

Circular soft magnetic dots are the main elements of many proposed novel spintronics devices, capable of fascinating spin-based electronics applications, from extremely sensitive magnetic field sensors, to current-tunable microwave vortex oscillators. Here, we investigate static and broadband dynamic magnetization responses of vertically coupled Permalloy (Py) magnetic dots in the vortex state in layered nanopillars (experiment and simulations), which were explored as a function of in-plane magnetic field and interlayer separation. Under reduction of magnetic field from saturation for the field range just above vortexvortex ground state. We observe a metastable double vortex state for each of the dots. In this state, novel kinds of spin waves (Winter’s magnons along domain walls between vortex cores and half-edge antivortex) are excited. For dipolarly coupled circular Py(25 nm)/Cu(20 nm)/Py(25 nm) trilayer nanopilars of diameter 600 nm, a small in-plane field splits the eigenfrequencies of azimuthal spin wave modes inducing an abrupt transition between acoustic (in-phase) and optic (out-of-phase) kinds of the low-lying coupled spin wave modes. Qualitatively similar changes (although more gradual and at higher values of in-plane fields) occur in the exchange coupled Py(25 nm)/Cu(1 nm)/Py(25 nm) trilayer nanopillars. These findings are in qualitative agreement with micromagnetic dynamic simulations.

We present the synthesis of polymer embedded colloidal ordered assemblies, built from highly ordered superparamagnetic manganese iron oxide nanocrystals. Each assembly is wrapped into a thin polymer shell. In-depth characterization of the nanoparticles by TEM, SAXS, SQUID, and magnetophoresis indicates that these colloidal hybrids exhibit high mobilities in external magnetic fields, and that they could efficiently serve as contrast enhancers in magnetic resonance imaging.

Hydrogenated amorphous silicon thin films were deposited using a high pressure sputtering (HPS) system. In this work, we have studied the composition and optical properties of the films (band-gap, absorption coefficient), and their dependence with the deposition parameters. For films deposited at high pressure (1 mbar), composition measurements show a critical dependence of the purity of the films with the RF power. Films manufactured with RF-power above 80W exhibit good properties for future application, similar to the films deposited by CVD (Chemical Vapor Deposition) for hydrogenated amorphous silicon.

The detailed structural and electronic characterization of microcrystalline powders of new spinels Lix-Mn3-x-yFeyO 4 (0.4 ≤ x ≤ 1.33; 0 ≤ y ≤ 1.30), obtained by the >liquid mix> method, is reported. Compositional characterization was carried out by means of thermogravimetric analysis, energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy, and their structure was refined from neutron and Xray diffraction showing a cubic symmetry between 5 and 550 K, Space Group Fd3̄m. However, at temperatures above ca. 950 K, a reversible transformation, probably implying the formation of an ordered vacant phase, has been detected. The magnetic behavior, analyzed from neutron diffraction data and magnetization measurements, is interpreted considering an >incomplete> ferrimagnetic response, due to magnetic frustration in the B sites. Potential applications have been evaluated from the magnetocaloric, electrochemical and dielectric behavior in selected compositional ranges. © 2013 Elsevier B.V. All rights reserved.

A new methodology for the preparation of nanostructured magnetic thermoset materials without any kind of metal oxide or metal magnetic nanoparticles has been proposed. The present study focuses on the nanostructuration of thermoset materials based on poly(ionic liquid)s block copolymer and the subsequent production of magnetic nanostructured thermoset. Judicious selection of block copolymer, such as P2VP-b-PMMA, soluble in epoxy and its subsequent quaternization leads to a phase modification of the system, from totally soluble to a nanostructured system. Different degree of quaternization of the pyridine groups showed the range of quaternization values which allowed the nanostructuration of the thermoset. Magnetic material was obtained by anion exchange of the quaternized poly(ionic liquid) block copolymer without using any kind of metal oxide or metal magnetic nanoparticles. The different materials obtained by quaternization of the block copolymers, the anion exchange of the polymeric ionic liquid and thermoset materials were characterized by 1H nuclear magnetic resonance (1H NMR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), high resolution transmission electron microscopy (HR-TEM) and magnetic properties were measured by a superconducting quantum interference device (SQUID). © 2013 American Chemical Society.

We have synthesized ZnO nanoparticles capped with butanethiol, octanethiol, and dodecanethiol. In all of the cases, the capped ZnO nanoparticles exhibit ferromagnetic-like behavior up to room temperature whose intrinsic origin has been demonstrated by using both X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectroscopy (XAS) at the Zn K-edge. Using these tools, we have also determined that the occurrence of ferromagnetism does not critically depend on the details of the synthesis but on the formation of a pristine ZnS/ZnO interface. Within this interface, ferromagnetism is favored in those regions where the local order is closer to wurzite-like ZnO than to w-ZnS. The study of ZnS-ZnO films prepared by cosputtering clearly indicates that increasing the disorder of this interface weakens the onset of ferromagnetic behavior. © 2013 American Chemical Society.

We present resistivity, magnetization, and specific-heat measurements on flux-grown single crystals of NiBi3. We find typical behavior of a type-II superconductor, with, however, a sizable magnetic signal in the superconducting phase. There is a hysteretic magnetization characteristic of a ferromagnetic compound. By following the magnetization as a function of temperature, we find a drop at temperatures corresponding to the Curie temperature of ferromagnetic amorphous Ni. Thus, we assign the magnetism in NiBi3 crystals to amorphous Ni impurities. © 2013 American Physical Society.

Bimagnetic materials were fabricated by decorating the external surface of rattle-type hollow silica microspheres (which themselves contain metallic cobalt nanoparticles) with magnetite nanoparticles; thus, each magnetic substance was spatially isolated by the silica shell. The amount of magnetite decoration on the co-occluded hollow silica was varied from 1 to 17 mass %. Magnetic and electronic properties of the resulting bimagnetic materials were characterized by superconducting quantum interference device measurements and X-ray absorption spectroscopy, respectively. The ferrous iron in the bimagnetic sample was slightly more oxidized than in the magnetite reference, probably from some charge-transfer because of the SiO2 surface contact, although the overall oxidation state of the samples is very similar to that of magnetite. The temperature dependence of the sample magnetization recorded with Zero Field Cooling and Field Cooling resulted in blocking temperatures for the bimagnetic materials that were close to that of magnetite nanoparticles (176 K) and were lower than that for the bare Co-occluded hollow silica (which was above room temperature). Values of coercive force and exchange bias at 300 K became quite small after decoration with only minimal amounts of magnetite nanoparticles (1-3 mass %) and were lower than those of magnetite. This is the first example of enhancing superparamagnetism by spatial separation of both Co and magnetite magnetic nanoparticles using a thin wall of diamagnetic silica. © 2013 AIP Publishing LLC.

In this work we investigate the magnetic properties of iron oxide nanoparticles obtained by two-step synthesis (seeded-growth route) with sizes that range from 6 to 18 nm. The initial seeds result monocrystalline and exhibit ferromagnetic behavior with low saturation field. The subsequent growth of a shell enhances the anisotropy inducing magnetic frustration, and, consequently, reducing its magnetization. This increase in anisotropy occurs suddenly at a certain size (~10 nm). Electronic and structural analysis with X-ray absorption spectroscopy indicates a step reduction in the oxidation state as the particle reaches 10 nm size while keeping its overall structure in spite of the magnetic polydispersity. The formation of antiphase magnetic boundaries due to island percolation in the growing shells is hypothesized to be the mechanism responsible of the magnetic behavior, as a direct consequence of the two-step synthesis route of the nanoparticles.

We present resistivity, specific heat and magnetization measurements in high quality single crystals of HoBi, with a residual resistivity ratio of 126. We find, from the temperature and field dependence of the magnetization, an antiferromagnetic transition at 5.7 K, which evolves, under magnetic fields, into a series of up to five metamagnetic phases. © 2013 Elsevier Ltd. All rights reserved.

Magnetoresistance (MR) in half doped copper LaCu0.5Mn 0.5O3 manganese related perovskite, synthesized by means of a soft chemistry pathway, is reported. Characterization by means of Rietveld refinement from powder X-ray diffraction data, SAED, HREM, and EELS indicates a random occupation of Cu2+ and Mn4+ at the octahedral sites of an orthorhombic perovskite lattice. This disordered situation is in agreement to the absence of charge ordering. LaCu0.5Mn 0.5O3 exhibits antiferromagnetic behavior related to Mn4+-O-Mn4+ and Cu2+-O-Cu2+ interactions. Ferromagnetic interactions are also evident as a consequence of a disordered arrangement of Cu2+ and Mn4+ over the octahedral sites, being the coexistence of these ferromagnetic superexchange interactions related to the observed MR behavior at low temperature. © 2013 American Chemical Society.

Postsynthesis thermal treatments are key to promote crystallinity and reduce the defect density in solution-processed nanomaterials. In particular, the annealing atmosphere strongly influences the functional properties of ZnO nanowires (NWs) and specifically their performance as photoanodes in dye-sensitized solar cells (DSCs). We prepared vertically aligned ZnO NWs by a low-cost, high-yield, and up-scalable hydrothermal method and studied the effect of the postannealing atmosphere on their optoelectronic properties and on their performance as electrodes in DSCs. When annealing ZnO NWs under argon, instead of air, significantly higher photoluminescence (PL) UV emission and relatively lower defects-related visible PL emission were obtained. At the same time, Ar-annealing rendered ZnO NWs with higher electrical conductivities, as observed from single NW measurements using conductive-atomic force microscopy. Furthermore, DSCs based on ZnO NWs annealed in argon were characterized by 50% higher photocurrents than those obtained from air-annealed ZnO. As a result 30% efficiency increases were systematically obtained when using argon as the annealing atmosphere. These results are discussed within the framework of a multiple trapping model for transport and charge transfer, taking into account differences in the defect concentration introduced during the annealing. © 2013 American Chemical Society.

La1-xBixMn2O5 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) oxides are members of the RMn2O5 family. The entire series has been prepared in polycrystalline form by a citrate technique. The evolution of their magnetic and crystallographic structures has been investigated by neutron powder diffraction (NPD) and magnetization measurements. All the samples crystallize in an orthorhombic structure with space group Pbam containing infinite chains of Mn4+O6 octahedra sharing edges, linked together by Mn3+O5 pyramids and (La/Bi)O8 units. These units become strongly distorted as the amount of Bi increases, due to the electron lone pair of Bi3+. All the members of the series are magnetically ordered below TN = 25-40 K and they present different magnetic structures. For the samples with low Bi content (x = 0.2 and 0.4) the magnetic structure is characterized by the propagation vector k = (0,0,1/2). The magnetic moments of the Mn4+ ions placed at octahedral sites are ordered according to the basis vectors (Gx, Ay,0) whereas the Mn3+ moments, located at pyramidal sites, are ordered according to the basis vectors (0, 0, C z). When the content of Bi increases, two different propagation vectors are needed to explain the magnetic structure: k1 = (0,0,1/2) and k2 = (1/2,0,1/2). For x = 0.6 and 0.8, k2 is predominant over k1 and for this propagation vector (k2) the magnetic arrangement is defined by the basis vectors (Gx, A y,0) and (Fx, Cy,0) for Mn4+ and Mn3+ ions, respectively. © 2013 IOP Publishing Ltd.